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Friday, October 17, 2014

497 lbs of Glass

UAS Construction Update for 10/17/2014

497 lbs of Glass

How do you lift a 497 pound piece of glass 50 feet in the air? 

Very Carefully (Banda Bing).  

No, with four suction cups and a fork lift. 

But how can a suction cup lift 500 pounds?  The physics behind the suction cup is a little counter intuitive, but actually very simple.   

 2014 10-17 Suction Cup Physics not 

     Suction Cup Physics (not)

This is how I thought a suction cup worked, before I took engineering classes. You place the suction cup up against the window and then with a small pump suck the air out of the inside of the suction cup.  The more air you suck out of the cup, the greater the negative pressure created inside the cup force holding the cup onto the window.  I thought that the only limitation on the negative force you could create inside the cup was the power of the pump creating the suction force.

 2014 10-17 Suction Cup Physics 

Suction Cup Physics

This is how I understand a suction cup works, after I took engineering classes. You place the suction cup up against the window and then with a small pump suck the air out of the inside of the suction cup.  However, once you suck out all of the air, there is nothing left inside the cup, it’s a vacuum.  It does not matter how powerful the pump is you can’t suck out something that is not there.   Since there is nothing inside the suction cup there is any negative pressure, in fact there is no force at all.  So what is holding the suction cup on the window?  It is the air outside the suction cup that is pushing the cup onto the window. That force is usually one earth atmosphere.

Calculating how much one suction cup can lift is just the force (one atmosphere) times the surface area (area of the suction cup) which can be expressed in the formula  F=PA

L = Load that a suction cup can hold

P = Pressure = Atmospheric pressure in Juneau at 9:00 am today is 995 mb = 0.98 Standard Atmosphere = 29.38 in-Mercury = 14.43 pounds/square inch

A = (Pi)(Radius Squared) = (3.14)(11)(11) = 95 square inches

L = (14.43 pounds/square inch)(95 square inches) =  1,370 pounds!

 

 

    2014 10-17 Suction Cup Window Lifter 

Suction Cup Window Lifter

The Contractor is using a suction cup window lifter by www.powrgrip.com  This window lifter has four suction cups attached to adjustable arms connected to a center pivot bearing.  This allows workers to attach the lifter to the window lying on its side and then lift it and rotate it into position. 

    2014 10-17 Rotating Window 

Rotating Window

 

You may think that with four suction cups we could lift (4)(1370) = 5,480 pounds.  However, the safety rating on this window lifter is only 700 pounds.  Just like most things in life, there are deductions.  Suction Cup Window Lifters are no different. There are deductions for; changes in shape of the suction cup, non-perpendicular lifting forces, strength of suction cup materials, changes in atmospheric pressure and safety of workers and by standards.

 2014 10-17 Lifting 497 pound window 

Lifting a 497 pound window

 

 2014 10-17 Placing 497 pound window 

Placing 497 pound window

  2014 10-17 Another Wonderful View 

Another Wonderful View from the UAS Freshman Housing

 

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